 ;; which take a set of keys naming components in the system to be
 ;; started/stopped. Order of the keys doesn't matter here.
 
+(def example-system-components [:scheduler :app :db])
+
 (defrecord ExampleSystem [config-options db scheduler app]
   component/Lifecycle
   (start [this]
-    (component/start-system this [:scheduler :app :db]))
+    (component/start-system this example-system-components))
   (stop [this]
-    (component/stop-system this [:scheduler :app :db])))
+    (component/stop-system this example-system-components)))
 
 ;; When constructing the system, specify the dependency relationships
-;; among components using the `using` function.
+;; among components with the `using` function.
 
 (defn example-system [config-options]
   (let [{:keys [host port]} config-options]

      (assoc component :connection conn)))

  (stop [component]
    ;; In the 'stop' method, shut down the running
    ;; component and release any external resources it has
    ;; acquired.

  component/Lifecycle

  (start [component]
    (println ";; Starting database")
    ;; In the 'start' method, initialize this component
    ;; and start it running. For example, connect to a
    ;; database, create thread pools, or initialize shared
